Charge Exchange


Between 2018 and 2022, I worked with Drs. George Clark and Peter Kollmann at the Applied Physics Lab (APL) to better understand the charge exchange process in Saturn's magnetosphere. Our work resulted in the following publication:

Sontag, A., Clark, G., & Kollmann, P. (2021). Charge exchange ion losses in Saturn's magnetosphere. Journal of Geophysical
           Research: Space Physics, 126, e2021JA029310.

Image taken from Figure 4 of Sontag et al. (2021)


Charge exchange is the process by which hot, positively-charged ions strip electrons from cold neutral gasses. This results in a hot (energetic) neutral atom (ENA) and a cold ion. The ENA is no longer bound by the planet's magnetic field, and quickly escapes the magnetosphere. The cold ion is below the energy threshold for most instruments' ion detection. Thus, charge exchange acts as a loss process for detectible ions in the presence of neutral gasses.

The goal of our research was to determine whether or not the observed ion losses in Saturn's magnetosphere could be (at least dominantly) accounted for by charge exchange. To explore this, we used data taken by the Cassini CHEMS instrument between 2004 and 2017 to form snapshot ion intensity profiles, developed a physical model of charge exchange that could take such profiles as initial conditions, and compared the outputs of the model to subsequent snapshots. We concluded that, for H+ and HnO+ ions between 5 and 15 Saturn radii with energies from 3 to 220 keV, charge exchange does indeed dominate the observed losses. This also allows us to make more precise conclusions about the density of Saturn's neutral torus.

The picture above shows our qualitative categorization of measured ion intensity profiles, along with our hypothesized physical paths between categories. Charge exchange takes profiles along the blue arrows, acting as a loss process. Fresh injections (see the cited paper) take profiles along the red arrows, acting as a source process. As injections age, the source regions fills out the profile evenly, sending profiles back to category 1.